Polarized displays, especially those making use of the unique properties of liquid crystalline materials, are employed in a wide variety of applications. Common polarized display systems are generally configured with two fixed polarizers sandwiching two glass plates on either side of a selectively polarizing material in combination with an array of electrodes for applying electrical fields to selected areas of the selectively polarizing matrix. The selectively polarizing material is typically one which exhibits optical birefringence by which different polarizations of light in the material can propagate through the material at different rates. This allows the polarization state of the incident light to be selectively altered to other polarization states. The polarizing material can include attenuation means, in which rays of one polarization state are selectively attenuated relative to rays of another polarization state.
A liquid crystal display, also known as an LCD, is a well-known example of a polarized display device. Liquid crystals have two principal properties making them useful in electronic display devices: (1) birefringence and (2) selective director alignment. The birefringence property of liquid crystal materials causes light passing through a specimen of one of these materials to change its plane of polarization as it passes through. For example, the plane of polarization may be rotated. The selective director alignment property of these materials causes the director, or locally averaged orientation of the anisotropic molecules in the liquid crystal material, to temporarily align itself with an applied orienting field such as an electric field.
Polarized liquid crystal displays employ these two properties in concert to selectively transmit or block light incident to selected areas of the display screen as desired. In the typical LCD device, incident light is polarized by a first polarizer to a first polarization state. As the light rays pass through the birefringent liquid crystal material, the polarization of the light rays in a given region is altered to a second polarization state. This change of polarization state can take the form of a simple rotation of the polarization but can also result in other polarization configurations such as elliptical polarization. As the light exits through an exit polarizer, light rays not having the third polarization state of the exit polarizer are absorbed. When the applied electric field is changed, the corresponding second polarization state is also changed, and the amount of light passing through the exit polarizer can therefore be controlled. Numerous polarized LCD modes have been developed, including twisted nematic (TN), super-twisted nematic (STN), ferroelectric, antiferroelectric, cholesteric, phase-change, guest-host, vertical-alignment (VA), in-plane switching (IPS), electrically controlled birefringence (ECB), pi-cells and many others. Devices incorporating the above described principles are disclosed in my U.S. Pat. No. 5,751,388, High Efficiency Polarized Display, and my U.S. Pat. No. 5,999,239, Method for Making a Polarization Sensitive Optical Element, which are herein incorporated by reference.
Polarized displays often make use of backlighting to improve the sharpness and brightness of the image thereon. In a backlit polarized display incorporated into a portable computing device, the efficiency of the display is at a premium, due to the demand for extended operation and limited battery power. The bulk of the power consumption of a backlit display is used for the generation of light. Both the efficiency and performance of the polarized display device are directly proportional to the efficiency with which the device concentrates and directs the generated light in a useful manner.
One device known to be useful in increasing image brightness is an optical film. Optical films are used in liquid crystal display systems and in other applications where control over the direction or polarization of light transmitted or reflected is desired in order to increase the brightness of a display. It is known that the luminance within a desired range of viewing angles of a polarized display can be significantly increased through the use of such an optical film.
Essentially, optical films comprise films of light transmissible materials that are designed to redirect light through reflection, refraction, scattering or a combination of these. When used in conjunction with a display screen, an optical film can significantly increase the brightness of a display within a certain desired range of viewing angles by controlling and redirecting light that would otherwise be absorbed or escape from the display at undesirable angles. Light that would otherwise be wasted is reflected or otherwise redirected back into the display or light source where a portion of it can be “recycled” and returned back to the film at an angle or polarization that allows it to escape from the display. This recycling is useful because it can reduce power consumption needed to provide a display with a desired level of brightness. The following patents and PCT publications describe various optical films, which are incorporated herein by reference:
U.S. Pat. No. 5,783,120 (PCT Publication WO 97/32222), A Method for Making an Optical Film;
PCT Publication WO 97/32223, An Optical Film with Co-Continuous Phases;
U.S. Pat. No. 5,825,543, Diffusely reflecting polarizing element including a first birefringent phase and a second phase;
PCT Publication WO 97/32224, An Optical Film;
PCT Publication WO 97/32225, Light Fixture Containing Optical Film;
PCT Publication WO 97/32226, Brightness Enhancement Film;
PCT Publication WO 97/32227, Optical Film with Increased Gain at Non-Normal Angles of Incidence; and
PCT Publication WO 97/32230, Optical Fiber with Light Extractor.
Test data from the foregoing types of films, however, clearly show that performance of these films is best for viewing angles close to the vector normal to the display screen. At wider viewing angles, it is known that the performance of such films suffers. There remains in the field a need for display devices having display properties and efficiency superior to those produced by the traditional architecture.